This invention relates to controls for automotive air conditioners driven by electric motors. In recent years, the percentage of American automobiles having air conditioning systems installed has increased to approximately 85%. Concurrently, efforts to improve fuel economy has resulted in smaller engines. In current automotive air conditioning systems, the compressor is belt driven from the engine crank shaft and is therefore constrained to follow engine speed. The load from a cycling fixed displacement compressor can significantly affect vehicle driveability and performance.
Current air conditioning systems are operated from the engine via belt drive, and control cooling capacity by either electrically cycling the compressor drive clutch on or off, or by throttling the compressor suction mechanically with a pressure activated control valve. Either way, evaporator pressure is maintained in a range which produces adequate cooling temperatures without being so cold that ice forms on the evaporator coil.
Another way to control cooling capacity is to mechanically vary compressor displacement over a continuous range to maintain constant evaporator pressure. Other variations of this recently commercialized approach include compressors that can operate on either all or half of their cylinders for 100%/50% capacity control. In all cases the evaporator pressure/temperature is maintained nearly constant and the temperature of air entering the vehicle is controlled by mixing heated air from the heater core with the cold air leaving the evaporator. The air entering the evaporator can either be outside air or recirculated cabin air, or a combination of the two depending on the particular vehicle design. A "maximum A/C" control is usually provided which limits outside air intake to approximately 15% of full flow.
In vehicles equipped with automatic temperature control (ATC) the temperature of air entering the vehicle is controlled by mixing heated air from the heater core with cold air leaving the evaporator. Control of the mixing door is accomplished by sensing cabin temperature and comparing it to the desired control setting. On fully automatic systems the fan speed is also controlled in proportion to the difference between actual and desired cabin temperature settings. More sophisticated ATC systems recently developed by the Japanese use a computer to predict vehicle cooling load based on measurement ambient and interior temperatures and solor insolation. The required cold air delivery temperature necessary to meet the load is calculated and the compressor is cycled at the proper frequency to maintain this temperature. The system saves energy by eliminating reheat at the expense of increased compressor cycling.
The conventional systems described above have several drawbacks. Use of the heater core to reheat conditioned air is inefficient. Cycling compressors result in wide fluctuation of air delivery temperatures. Compressors that mechanically modulate capacity do not control air delivery temperature, but instead maintain a constant evaporator temperature. This allows the blower fan speed and outside air intake to adversely affect delivery air temperature and hence lower comfort levels.
U.S. Pat. No. 4,459,519 shows a refrigeration system for use in the passenger compartment of a vehicle. The system comprises a brushless D.C. motor hermetically sealed with a compressor. Compressor speed is varied by sensing evaporator temperature and comparing it with a reference temperature. An alternator output is controlled in accordance with the difference between the evaporator temperature and the reference temperature, thereby varying the speed of the brushless D.C. motor. The temperature of the compartment being cooled is sensed and compared with a desired compartment temperature, and the operating speed of the evaporator fan is varied in accordance with the difference between the sensed temperature and the desired temperature. The reference temperature for the evaporator is automatically varied in accordance with a signal indicative of the evaporator fan speed, thereby changing the temperature of the air moved by the fan, facilitating control of the compartment temperature at the desired temperature.
It is an object of the present invention to provide a control for an electrically driven automotive air conditioner that can provide full cooling capacity at any engine speed.
It is a further object of the present invention to provide a control for an electrically driven automotive air conditioner that can provide control of air conditioned air temperature without using reheating or compressor cycling.
It is a still further object of the present invention to provide a control for an electrically driven automotive air conditioner that protects the electric motor driving the compressor from overload conditions while maintaining driver comfort.